Intermodulation products may be generated in a wireless system when two or more signals at different frequencies are transmitted along a signal path including a component having a non-linear transmission characteristic; these products differ in frequency from the signals from which they were generated, and may potentially cause interference to other signals. The generation of intermodulation products is becoming a problem of increasing importance in modern wireless communication systems, and in particular cellular wireless systems, since the radio frequency spectrum available has been steadily expanded as additional bands have become available, and the pattern of allocation of uplink and downlink bands within the available spectrum for use by various cellular systems, such systems using GERAN (GSM EDGE Radio Access Network), UTRAN (UMTS Terrestrial Radio Access Network) and E-UTRAN (Evolved UMTS Terrestrial Radio Access Network) radio access networks, and by various operators, is complex and territorially dependent. In this environment, and in particular at a cellular radio base station, it is likely that circumstances arise in which intermodulation products generated from transmitted carriers in one or more downlink bands would fall within an uplink band in which signals are received at the base station. Intermodulation generated by non-linear characteristics of active components such as power amplifiers may generally be dealt with at a design stage by appropriate frequency management and filtering, but intermodulation caused by non-linear characteristics of passive components, so called passive intermodulation (PIM), may prove more difficult to manage. Many passive components may exhibit a non-linear transmission characteristic to some degree, for example due to an oxide layer at a metal to metal contact, and the non-linear characteristic may develop with time as an ageing process of the component. Furthermore, PIM may be caused by components in the signal path between the transmitter and the receiver that are external to the transceiver equipment and which may be outside the operator's control, such as metallic objects on the antenna tower or other objects in the signal path through the propagation environment, such as fences; this is known as the “rusty bolt” effect.
Interference due to PIM may reduce the carrier to interference ratio at a receiver, which may reduce coverage by a cell significantly. Conventionally, as a solution to the problem of suspected interference caused by PIM in a cellular radio network, the source of the PIM may be tracked down by field engineers and a component causing the PIM may be replaced. However, this is labour intensive and relies on the problem being identified in order to alert the field engineers.
Automatic systems have been proposed for cancelling intermodulation products in the field of satellite communications, in which intermodulation products generated within a transmitter or receiver are cancelled using cancellation signals generated from baseband signals representing the carriers. In one version, a delay may be applied to the cancellation signals to align the cancellation signals with transmitter signals, so that intermodulation products may be cancelled from the transmitter signals before transmission from an antenna. The source of the intermodulation products in this version is assumed to be located at the transmitter, and the delay may be adjusted on the basis of a tone that is inserted into the transmitter signals. However, in many systems it is undesirable to modify a transmit signal by the use of a tone, and the source of the intermodulation products may not be known, so that the use of a tone may not be appropriate since it may be limited to adjustment of delay over a narrow range of values if aliasing between delay values is to be avoided.
In many wireless systems, for example cellular radio networks, a wide range of delay values is possible between the source of intermodulation products and a receiver, since the source of intermodulation products, and in particular PIM, may be unknown, and may be located at an any point in the transmit path. Also, it may be the case that signals which combine to produce intermodulation products may originate from different locations, and may experience different delays between a point at which they may be sampled and the location of the source of intermodulation products. Furthermore, in Multiple Input Multiple Output (MIMO) systems, signals that combine to produce intermodulation products may each be composed of signal components at the same frequency originating at different locations, typically different transmit antennas, so that the signal components may each experience a different delay.
A system for cancellation of intermodulation products may suffer from limited cancellation performance as a result of differential delays between a cancellation signal and a signal to be cancelled, due to changing signal content with time and due to a phase slope across the bandwidth of the signals. This is especially relevant to modern cellular radio systems such as LTE (Long Term Evolution) which may typically occupy, for example, 20 MHz bandwidth. Furthermore, limited performance will also result from differential delays between components of the signal, such as MIMO components with respect to the corresponding components of the cancellation signal.
Aspects of the invention address at least some of the limitations of the prior art systems.